The invention relates generally to a copper-based alloy and an article containing the same.
More particularly, the invention relates to a corrosion-resistant copper-based alloy and an article including such alloy.
Corrosion-resistant copper-based alloys are employed for the facades of buildings, for roofs, for the gutters of buildings and for components subjected to the corrosive effects of flowing media. These alloys are exposed to a variety of corrosive actions. One such action is general surface corrosion. This type of corrosion, which affects virtually all copper-based alloys, depends upon the composition of the alloy and usually decreases with time.
Another type of corrosion known as stress-corrosion cracking can occur in certain copper alloys, especially brasses, exposed to specific environments. Stress-corrosion cracking has been recently observed in copper water pipes.
Normal constituents of the atmosphere can constitute potent corrosive agents. Examples are chloride particles found in coastal air; moisture; and contaminants such as sulfur dioxide, carbon dioxide, hydrogen sulfide, ammonia and amines found in urban and industrial areas.
Corrosion can be classified into the following three categories:
1. Dry corrosion which occurs indoors and in desert climes. Dry corrosion is characterized in that the atmosphere is practically devoid of water vapor. In the absence of airborne contaminants, invisible oxide films form on copper and its alloys at room temperature. These films become visible at elevated temperatures. If the air is moderately contaminated, a visible film forms on copper and its alloys even at room temperature. This is known as tarnishing.
2. Moist corrosion which arises when the amount of water vapor in the atmosphere exceeds a lower threshold value referred to as the critical moisture content. Gaseous or solid contaminants accelerate the corrosion rate and frequently lead to a reduction in the critical moisture content.
3. Wet corrosion which is associated with rain and is of great significance in our climate. Generally speaking, the aggressiveness of rain increases as the amount of precipitation decreases. However, rain can also have a beneficial influence on the weather-resistance of copper and its alloys. Thus, rain accelerates the formation of protective films and, in addition, washes away dust, soot and acidic compounds all of which can increase the corrosive rate or affect the surface appearance of copper and copper-based alloys.
If copper is exposed to an atmosphere containing traces of hydrogen sulfide, corrosion proceeds rapidly. The surface film which forms consists of a mixture of copper sulfide and copper oxide with the latter predominating. The film thickness increases parabolically as is the case for the oxidation of copper in clean, dry air. However, the reaction rate is far greater in the presence of sulfides.
The tarnishing rate is proportional to the sulfide content of the atmosphere. At very low sulfide concentrations, the presence of water vapor can inhibit the reaction. In this corrosive process, hydrogen sulfide is clearly the catalyst while the thickness of the tarnish film is the controlling or limiting factor.
As another example, consider the behavior of copper which is exposed to air containing water vapor and contaminated with sulfur dioxide.
In the complete absence of water vapor, sulfur dioxide has no influence on the oxidation of copper at room temperature and such oxidation proceeds as it would in dry, clean air. The metal is not noticeably attacked. However, in the presence of water vapor, the rate of attack depends upon the concentration of sulfur dioxide as well as the relative humidity. Curves of weight gain versus time exhibit a steep slope for the first few days and then level off towards the time axis so that they are almost parallel to such axis after about 30 days. The corrosion rate has a minimum at a sulfur dioxide concentration of approximately 1 percent. This is attributed to the composition of the corrosion products. At a concentration of 1 percent, the film on the copper consists of copper sulfate. Below this concentration, the film consists of basic copper sulfate whereas it consists of an acidic salt above this concentration.
Another striking fact is that the corrosion rate increases significantly at a relative humidity in excess of 63 percent. This phenomenon is ascribed to the hygroscopic characteristics of the film formed by the corrosion products. At a relative humidity above 63 percent, the film becomes capable of absorbing water vapor. The relative humidity threshold value above which the corrosion rate discontinuously jumps to a much higher value is known as the critical moisture content. Here, the amount of water vapor in the air is the controlling or limiting factor and the sulfur dioxide is the catalyst.
The precise mechanism by which the sulfur dioxide attacks the metallic surface is not clear in every case.
Wind, sun and temperature can play a significant role in atmospheric corrosion since they determine how rapidly the surface dries and how long it remains wet. However, a corrosive atmosphere is not adequately characterized by average values of temperature and moisture. The changes in these factors during the time that the material is exposed to the atmosphere are far more suitable for this purpose.
In general, air in industrial areas is considered to be more aggressive than ocean air although this depends upon the degree of contamination of the industrial air and the salt content of the ocean air.
As a rule, copper facades for buildings and copper roofing consist of phosphorus-deoxidized copper. This "pure" copper undergoes patina formation under atmospheric conditions. Patina is a natural protective film which shields the pure copper from the direct effects of weather.
A newly laid copper roof has a more or less marbled appearance. Its surface has dark spots which are generated when the tiles are gripped. After 6 to 12 months, the surface normally assumes a uniform dark brown color. The surface then generally maintains this appearance for several years. In the course of time, a green color may also develop.
The chemical composition of the patina depends upon the climate. In rural and normal urban atmospheres, the patina consists of basic copper sulfate with small amounts of a basic carbonate. At the ocean, the basic sulfate is partially replaced by a basic chloride. The latter can predominate in coastal regions which are remote from urban areas. Depending upon age, the basicity of the film reaches a maximum pH of 3. The corrosion products then consist of CuSO.sub.4.3Cu(OH).sub.2 which is known as brochantite, CuCO.sub.3.3Cu(OH).sub.2 which is known as malachite, and CuCl.sub.2 3 Cu(OH).sub.2 which is known as atacamite. The protective characteristics of the patina are due to the stability of these compounds.
The degree of contamination, existing winds and rain, as well as the shapes of the copper-containing structural elements and roof inclination, can all influence patina formation.
As a rule, a dense patina develops rapidly in highly contaminated industrial atmospheres. In moderately contaminated urban environments, the copper likewise becomes covered with a dense dark oxide film midway through its life.
The following erosion rates for copper due to atmospheric corrosion are given by W. Wiederhold for Mid and Western Europe (W. Wiederhold, "Die atmospharische Korrosion von Kupfer und Kupferlegierungen", Zeitschrift Werkstoffe und Korrosion, 15(1964), pp. 633-644):
______________________________________ Rural Air 1.9 micrometers per year Urban Air 1.5 to 2.9 micrometers per year Industrial Air 3.2 to 4.0 micrometers per year Ocean Air 3.8 micrometers per year ______________________________________
The erosion rates generally apply to the first few years. They decrease continuously with time and reach a value of virtually zero after approximately 70 years at which time the patina has achieved its greatest basicity.
The increasing environmental pollution by contaminants of all types has resulted in the approximate air pollution levels set forth in the appended Table 1 (excerpted from Technischer Uberwachungsverein Rheinland, 1983). The units "t/a.km.sup.2 " in Table 1 represent tons per annum per square kilometer. The footnotes to Table 1 are as follows:
(1) PA1 (2) Given as NO.sub.2 PA1 (3) Hydrocarbons and other organic compounds
A designates the southern Rheinschiene region PA2 B designates the western Ruhr region PA2 C designates the eastern Ruhr region PA2 D designates the central Ruhr region PA2 E designates Ludwigshafen/Frankenthal PA2 F designates Rhein-Main PA2 G designates Mainz
It is noted that the organic gases and vapors for traffic in the southern Rheinschiene region were calculated on the basis of emission factors obtained from infrared measurements.
TABLE I __________________________________________________________________________ Comparison of Emissions in the Air for Different Regions Region (1) A B C D E F G Emissions % t/a .multidot. km.sup.2 % t/a .multidot. km.sup.2 % t/a .multidot. km.sup.2 % t/a .multidot. km.sup.2 % t/a .multidot. km.sup.2 % t/a .multidot. km.sup.2 % t/a .multidot. km.sup.2 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 __________________________________________________________________________ Industry 1 Carbon 26 106 85.2 1940 49.9 358 28 183.4 50 287.7 0.5 1.5 1.4 2.1 Monoxide 2 Nitrogen 84.2 117 82.5 106 84.9 114 87.0 179.1 87 202.3 50.5 19.9 80.5 43.6 Oxides (2) 3 Sulfur 88.1 228 91.9 271 88.9 157 93.9 383 95.5 283 66.9 36.1 85.8 70.4 Dioxide 4 Organic 88.4 129.6 87.5 211.9 22.5 6.1 56 38.4 80 107.9 75.5 57.7 58.3 11.4 Gases and Vapors (3) Traffic 1 Carbon 42 166 5.9 143 22 157 28.5 186 34 198.6 65 185 61.9 96 Monoxide 2 Nitrogen 10.8 15.2 13.9 13.9 12.3 16.6 10.6 21.8 11 25.2 34.8 18.6 13.2 4.7 Oxides (2) 3 Sulfur 0.9 2.1 0.9 1.8 1 1.8 0.4 1.7 0.5 1.3 2.7 1.9 1.1 0.9 Dioxide 4 Organic 5 7.3 5.3 12.7 52.4 14.3 26.1 17.9 14 18.4 16.5 12.6 18.2 3.6 Gases and Vapors (3) Home Heating Fuel Small Businesses 1 Carbon 32 130 8.9 200 28.1 200 43.5 285.1 16 94.5 34.5 97 36.7 57 Monoxide 2 Nitrogen 5 6.9 3.6 4.6 2.8 3.7 2.4 5 2 6 14.7 5.7 6.3 3.4 Oxides (2) 3 Sulfur 11 28 7.2 21 10.1 18 5.7 23.1 4 11.9 30.4 16 13.1 11 Dioxide 4 Organic 6.6 9.7 7.2 17.4 25.1 6.8 17.9 12.2 6 8.3 8 6.1 23.5 4.6 Gases and Vapors (3) __________________________________________________________________________
For the past several years, air pollution has been associated with "acid rain" which has the approximate composition set forth in Table 2 below (excerpted from "Der Rat von Sachverstandingen fur Umweltfragen: Waldschaden und Luftverunreinigung", Sondergutachten, 1983, Verlag Kohlhammer, Stuttgart/Mainz).
TABLE 2 ______________________________________ Concentration of Pollutant Ions in Rain Water for Different Amounts of Precipitation Amount of Precipitation NH.sub.4.sup.+ NO.sub.3.sup.- SO .sub.4.sup.-- Cl.sup.- (mm) (g/m.sup.3) (g/m.sup.3) (g/m.sup.3) (g/m.sup.3) ______________________________________ 0.3 13.1 9.3 58.4 17.5 0.3-1.0 5.0 5.8 30.0 7.1 1.1-3.0 2.6 2.6 19.8 3.4 3.1-7.0 1.9 2.0 9.4 1.7 7.1-11.0 1.6 1.7 7.0 1.4 11.0 1.9 1.2 5.8 1.3 ______________________________________
At high pollutant concentrations, atmospheric and roof water develop an acidity, e.g., by the formation of a "dilute acid" consisting of HCl and H.sub.2 SO.sub.4, which exceeds the tendency of the copper to form soluble compounds. Thus, the initially thin patina which forms on newly laid roofs is locally destroyed since the substances constituting the patina, e.g., CuSO.sub.4 and CuCO.sub.3, are no longer stable under the influence of the dilute acid. The copper is accordingly unprotected at these localized areas and is exposed to the permanently acting pollutants. The destructive corrosion proceeds locally and leads to pitting.